The strains in the vaccine are only an estimate.
Since there are so many strains of the flu, it is impossible to estimate with 100% accuracy, which strains should be in the vaccine. There is actually a lot of work that goes into these decisions. For instance, the CDC conducts research and uses a complex algorithm in order to try and understand by taking data pertaining to which strands of the flu are infecting people around the world. This allows for a prediction of which will impact those in the United States. Again, this is only an estimate and there is no way to know with complete confidence which strains should be included in the flu shot.
As we mentioned before, the flu is mutating almost constantly and this data is collected in the beginning of the year, before flu season starts. Therefore, this may lead to a lesser chance of knowing for certain which strains will be the most prevalent.
Again, flu shots include at least three different strains of the flu, which increases the likelihood of being able to protect us against different strains of the flu. Also, the antibodies produced by your body to protect against one strain of the flu can also help protect against other strains in some cases. This process is known as cross-protection. .
